Temperature sensor circuit and semiconductor device including temperature sensor circuit

ABSTRACT

To provide a highly accurate temperature sensor circuit. The temperature sensor circuit includes a first constant current circuit; a first diode in which a first voltage reflecting the temperature of an object to be detected is generated between an anode and a cathode in accordance with a first current supplied from the first constant current circuit; a second constant current circuit; a second diode which includes an oxide semiconductor and in which a second voltage is generated between an anode and a cathode in accordance with a second current supplied from the second constant current circuit; and an amplifier circuit which amplifies a difference between the first voltage and the second voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An embodiment of the present invention relates to a temperature sensorcircuit including a semiconductor element and a semiconductor deviceincluding the temperature sensor circuit.

2. Description of the Related Art

A temperature sensor circuit includes a sensor which convertstemperature data into an electric signal (sensor signal) and circuitsfor processing the sensor signal output from the sensor. In the casewhere a temperature sensor circuit is formed in a semiconductorintegrated circuit, not a resistance thermometer bulb, a thermistor, athermocouple, or the like but a sensor utilizing the influence oftemperature of a diode is generally used.

Specifically, in the case of a temperature sensor circuit including adiode as a sensor, with the diode where a proportion of variations inelectrical characteristics depending on the temperature (i.e., theproportion is influence of temperature and the proportion is also calledtemperature dependence) is large, temperature data of an object to bedetected can be obtained using a forward voltage generated when aforward current is constant or a forward current when a forward voltageis constant. For example, a forward voltage generated when a constantforward current flows from a constant current source to the diode islower when the temperature of the diode is higher, and is higher whenthe temperature of the diode is lower. Thus, the forward voltage of thediode depends on the temperature of the diode.

Patent Document 1 below discloses a temperature sensor including aresistor serving as a current control element and a diode serving as asensor element; the resistor and the diode are connected between a powersource VDD and ground GND.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2011-007545

SUMMARY OF THE INVENTION

In the case of the temperature sensor circuit having the aboveconfiguration, even when temperature is constant, measurement valuevaries unless a current or a voltage supplied to the diode is keptconstant. When a too high current flows to the diode, the diodegenerates heat, which causes a difference in the temperature between anobject to be detected and the diode. For this reason, the temperaturesensor circuit having the above configuration needs to be provided witha constant current circuit or a constant voltage circuit so that a lowcurrent of approximately several μA to several hundred μA which is keptconstant can be supplied to the diode to measure the temperature of anobject to be detected with high accuracy.

However, a constant current circuit or a constant voltage circuitgenerally includes a transistor including silicon in a channel formationregion. In a transistor including silicon in a channel formation region,as the temperature increases, the drain current increases due to a shiftin threshold voltage. Thus, when the temperature of a temperature sensorcircuit is higher, a current which is output from a constant currentcircuit or a voltage output from a constant voltage circuit is morelikely to vary due to a fluctuation in the threshold voltage of thetransistor. When the electrical characteristics of the transistorincluded in the constant current circuit or the constant voltage circuitvary, the current or voltage thereof also varies. The forward voltage orforward current of a diode is influenced by a small fluctuation insupplied current or voltage; therefore, a fluctuation in current outputfrom the constant current circuit or voltage output from the constantvoltage circuit makes it difficult to measure the temperature of anobject to be detected with high accuracy.

In view of the above technical background, an object of one embodimentof the present invention is to provide a highly accurate temperaturesensor circuit. Another object of one embodiment of the presentinvention is to provide a semiconductor device in which accuratetemperature data obtained with the use of the temperature sensor circuitcan be utilized for the operation.

In one embodiment of the present invention, a temperature sensor circuitincludes a first semiconductor element for obtaining temperature dataand a second semiconductor element receiving a small influence oftemperature. In another embodiment of the present invention, the secondsemiconductor element includes an oxide semiconductor. A semiconductorelement including an oxide semiconductor receives a smaller influence oftemperature of a voltage generated between terminals than a transistorincluding a normal semiconductor such as silicon or germanium in achannel formation region. Thus, the voltage between terminals of thesecond semiconductor element is less likely to be affected by thetemperature of an object to be detected.

A voltage or a current generated between terminals of the firstsemiconductor element by supply of a current or a voltage is comparedwith a voltage or a current generated between terminals of the secondsemiconductor element by supply of a current or a voltage, wherebytemperature data of an object to be detected is obtained.

In another embodiment of the present invention, with the aboveconfiguration, a factor which affects a voltage generated betweenterminals of the first semiconductor element and which is other than thetemperature of an object to be detected can be prevented frominfluencing a measurement value, so that more accurate data of thetemperature of the detected object can be obtained; the factor is, forexample, a variation with temperature in a current supplied from theconstant current circuit or a voltage supplied from the constant voltagecircuit, or variations in the electrical characteristics of thetransistor included in the constant current circuit or the constantvoltage circuit.

According to one embodiment of the present invention, a highly accuratetemperature sensor circuit can be provided. According to anotherembodiment of the present invention, a semiconductor device in whichaccurate temperature data obtained with the use of the temperaturesensor circuit can be utilized for the operation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates the configuration of a temperature sensor circuit;

FIG. 2A illustrates the configuration of a temperature sensor circuit,and FIG. 2B is a cross-sectional view of a transistor;

FIG. 3 illustrates the configuration of a temperature sensor circuit;

FIG. 4 illustrates the configuration of a temperature sensor circuit;

FIGS. 5A and 5B each illustrate the configuration of a constant currentcircuit;

FIG. 6 illustrates the configuration of a semiconductor device

FIG. 7 illustrates the configuration of a semiconductor device;

FIGS. 8A to 8D are each a cross-sectional view of a transistor;

FIG. 9 is a cross-sectional view of a temperature sensor circuit;

FIGS. 10A and 10B are each a graph showing the measured values of draincurrent versus gate voltage; and

FIG. 11A is a graph showing variations in shift values versus substratetemperature and 11B is a graph showing variations in subthreshold swingsversus substrate temperature.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description and it iseasily understood by those skilled in the art that the mode and detailscan be variously changed without departing from the scope and spirit ofthe present invention. Accordingly, the invention should not beconstrued as being limited to the description of the embodiments below.

Note that a temperature sensor circuit of one embodiment of the presentinvention can be employed for a wide variety of semiconductor devicessuch as integrated circuits, RF tags, and semiconductor display devices.The semiconductor devices including temperature sensor circuits areincluded in the category of the present invention. The integratedcircuits include, in the category, large scale integrated circuits(LSIs) including a microprocessor, an image processing circuit, adigital signal processor (DSP), and a microcontroller and programmablelogic devices (PLDs) such as a field programmable gate array (FPGA) anda complex PLD (CPLD). The semiconductor display devices include, in thecategory, liquid crystal display devices, light-emitting devices havingpixels each provided with a light-emitting element typified by anorganic light-emitting element (OLED), electronic paper, digitalmicromirror devices (DMDs), plasma display panels (PDPs), and fieldemission displays (FEDs).

(Embodiment 1)

FIG. 1 illustrates a configuration example of a temperature sensorcircuit of one embodiment of the present invention. The temperaturesensor circuit 100 in FIG. 1 includes a semiconductor element 101, asemiconductor element 102, a constant current circuit 103, a constantcurrent circuit 104, and an amplifier circuit 105.

The semiconductor element 101 includes an oxide semiconductor and isserially connected to the constant current circuit 103 between a nodesupplied with a low-level potential VSS and a node supplied with ahigh-level potential VDD. Specifically, a first terminal of thesemiconductor element 101 is connected to the node supplied with thepotential VSS, and the constant current circuit 103 is connected betweenthe second terminal of the semiconductor element 101 and the nodesupplied with the potential VDD.

The semiconductor element 102 includes a semiconductor where theinfluence of temperature is larger than that of an oxide semiconductor,such as single crystal silicon, polycrystalline silicon,microcrystalline silicon, or amorphous silicon; however, one embodimentof the present invention is not limited thereto. The semiconductorelement 102 and the constant current circuit 104 are connected in seriesbetween the node supplied with the potential VSS and the node suppliedwith the potential VDD. Specifically, a first terminal of thesemiconductor element 102 is connected to the node supplied with thepotential VSS, and the constant current circuit 104 is connected betweena second terminal of the semiconductor element 102 and the node suppliedwith the potential VDD.

Note that FIG. 1 illustrates the case where the semiconductor element101 and the constant current circuit 103 are connected in series and thesemiconductor element 102 and the constant current circuit 104 areconnected in series, between the node supplied with the potential VSSand the node supplied with the potential VDD. However, the semiconductorelement 101 and the constant current circuit 103 may be connected inseries between a node supplied with a low-level potential VSS 1 and anode supplied with a high-level potential VDD1 and the semiconductorelement 102 and the constant current circuit 104 may be connected inseries between a node supplied with a low-level potential VSS2 differentfrom the potential VSS1 and a node supplied with a high-level potentialVDD2 different from the potential VDD1.

A voltage generated between the first terminal and the second terminalof the semiconductor element 101 when a predetermined forward currentI₁₀₁ flows from the constant current circuit 103 to the semiconductorelement 101 is a forward voltage V₁₀₁. The semiconductor element 101includes an oxide semiconductor and thus receives a small influence oftemperature. For this reason, the forward voltage V₁₀₁ is less likely tobe influenced by the temperature of the semiconductor element 101, thatis, the forward voltage V₁₀₁ is less likely to reflect the temperatureof the semiconductor element 101.

A voltage generated between the first terminal and the second terminalof the semiconductor element 102 when a predetermined forward currentI₁₀₂ flows from the constant current circuit 104 to the semiconductorelement 102 is a forward voltage V₁₀₂. The semiconductor element 102receives a larger influence of temperature than the semiconductorelement 101. Thus, the higher the temperature of the semiconductorelement 102, the lower the forward voltage V₁₀₂. Specifically, in thecase of the semiconductor element 102 including silicon, the forwardvoltage V₁₀₂ varies at a rate of approximately −2 mV/° C. Thus, theforward voltage V102 reflects the temperature of an object to bedetected.

The amplifier circuit 105 has a function of amplifying a voltagedifference between the forward voltage V₁₀₁ and the forward voltage V₁₀₂and outputting the amplified voltage difference as a voltage Vout froman output terminal 110 of the amplifier circuit 105. Specifically, inthe temperature sensor circuit 100 illustrated in FIG. 1, the potentialof the second terminal of the semiconductor element 101 obtained byadding the forward voltage V₁₀₁ to the potential VSS and the potentialof the second terminal of the semiconductor element 102 obtained byadding the forward voltage V₁₀₂ to the potential VSS are supplied to theamplifier circuit 105. As the amplifier circuit 105, a differentialamplifier circuit can be used, for example.

Note that the forward voltage V₁₀₂ contains, as data, a variation withtemperature in the forward current I₁₀₂ supplied from the constantcurrent circuit 104, variations in electrical characteristics of atransistor included in the constant current circuit 104, and the likebesides the temperature of the semiconductor element 102. The forwardvoltage V₁₀₁ presumably does not contain data of the temperature of thesemiconductor element 101 but contains a variation with temperature inthe forward current I₁₀₁ supplied from the constant current circuit 103,variations in the electrical characteristics of a transistor included inthe constant current circuit 103, and the like as data. Thus, assumingthat there is no difference in influence of temperature, the electricalcharacteristics of the transistors, and the like between the constantcurrent circuit 103 and the constant current circuit 104, thedifferences between the constant current circuit 103 and the constantcurrent circuit 104 in influence of temperature, the electricalcharacteristics of the transistors, and the like are canceled in thevoltage Vout obtained when the amplifier circuit 105 amplifies thevoltage difference between the forward voltage V₁₀₁ and the forwardvoltage V₁₀₂. Thus, in the temperature sensor circuit 100 of oneembodiment of the present invention, a factor which affects the forwardvoltage V₁₀₂ of the semiconductor element 102 and which is other thanthe temperature of an object to be detected can be prevented frominfluencing a measurement value the voltage Vout, so that more accuratedata of the temperature of the detected object can be obtained; thefactor is, for example, a variation with temperature in a currentsupplied from the constant current circuit 104 to the semiconductorelement 102 or variations in the electrical characteristics of thetransistor included in the constant current circuit 104.

Note that the forward current I₁₀₁ supplied to the semiconductor element101 does not necessarily have to be equal to the forward current I₁₀₂supplied to the semiconductor element 102. However, when the forwardcurrent I₁₀₁ is equal to or substantially equal to the forward currentI₁₀₂, differences between the constant current circuit 103 and theconstant current circuit 104 in influence of temperature, the electricalcharacteristics of the transistors, and the like can be cancelled moreaccurately.

Next, FIG. 2A illustrates a specific configuration example of thetemperature sensor circuit 100 in FIG. 1.

In the temperature sensor circuit 100 in FIG. 2A, a transistor 101 t isused as the semiconductor element 101 and a transistor 102 t is used asthe semiconductor element 102. Specifically, one of a source terminaland a drain terminal of the transistor 101 t is connected to a nodesupplied with the potential VSS, and a gate electrode and the other ofthe source terminal and the drain terminal of the transistor 101 t areconnected to the constant current circuit 103. The potential of theother of the source terminal and the drain terminal of the transistor101 t and the potential of the gate electrode of the transistor 101 tare supplied to the amplifier circuit 105. One of a source terminal anda drain terminal of the transistor 102 t is connected to the nodesupplied with the potential VSS, and a gate electrode and the other ofthe source terminal and the drain terminal of the transistor 102 t areconnected to the constant current circuit 104. The potential of theother of the source terminal and the drain terminal of the transistor102 t and the potential of the gate electrode of the transistor 102 tare supplied to the amplifier circuit 105.

Note that the temperature sensor circuit 100 may further include anothercircuit element such as a transistor, a diode, a resistor, or aninductor as needed.

Note that “source terminal” of a transistor in this specification refersto a source region that is part of a semiconductor film functioning asan active layer or a source electrode connected to the semiconductorfilm. Similarly, “drain terminal” of a transistor in this specificationrefers to a drain region which is part of a semiconductor filmfunctioning as an active layer or a drain electrode connected to thesemiconductor film.

Note that the terms “source terminal” and “drain terminal” of atransistor are interchanged depending on the type of the channel of thetransistor or the levels of potentials supplied to the source terminaland the drain terminal. In general, as for a source terminal and a drainterminal of an n-channel transistor, one to which a lower potential issupplied is called a source terminal, and one to which a higherpotential is supplied is called a drain terminal. Further, as for asource terminal and a drain terminal of a p-channel transistor, one towhich a lower potential is supplied is called a drain terminal, and oneto which a higher potential is supplied is called a source terminal. Inthis specification, although the connection relation of the transistormay be described assuming that the source terminal and the drainterminal are fixed for convenience, actually, the names of the sourceterminal and the drain terminal are interchanged depending on therelation of the potentials.

In the transistor 101 t, an oxide semiconductor is used for an activelayer. In the transistor 102 t, a semiconductor receiving a largerinfluence of temperature than the oxide semiconductor is used for anactive layer.

FIG. 2B illustrates an example of a cross-sectional structure of thetransistor 101 t. The transistor 101 t in FIG. 2B includes, over asubstrate 120 having an insulating surface, a semiconductor film 121serving as an active layer, a source electrode 122 and a drain electrode123 over the semiconductor film 121, a gate insulating film 124 over thesemiconductor film 121, the source electrode 122, and the drainelectrode 123, and a gate electrode 125 which is over the gateinsulating film 124 and between the source electrode 122 and the drainelectrode 123 and overlaps with the semiconductor film 121.

Further, an insulating film 126 is provided over the transistor 101 t,and a conductive film 127 connected to the gate electrode 125 and thedrain electrode 123 through an opening formed in the gate insulatingfilm 124 and the insulating film 126 is provided over the insulatingfilm 126.

In the transistor 101 t illustrated in FIG. 2B, a region of thesemiconductor film 121 which is between the source electrode 122 and thedrain electrode 123 and overlaps with the gate electrode 125 correspondsto a channel formation region 121 c. A region of the semiconductor film121 which overlaps with the source electrode 122 corresponds to a sourceregion 121 s, and a region of the semiconductor film 121 which overlapswith a drain electrode 123 corresponds to a drain region 121 d.

In one embodiment of the present invention, the oxide semiconductorneeds to be included in at least the channel formation region 121 c ofthe semiconductor film 121; however, the oxide semiconductor may beincluded in the whole of the semiconductor film 121.

Next, the values of drain current Id versus gate voltage Vg of atransistor where an oxide semiconductor film was used for an activelayer and a transistor where a single crystal silicon film was used foran active layer will be described; the values were measured whiletemperature was changed. Note that the gate voltage Vg refers to thevoltage of a gate electrode, using the potential of a source electrodeas a reference potential.

The measurement was performed under the conditions that a drain voltageVd is 0.1 V and the gate voltage Vg is in the range from −3 V to +3 V.Note that the drain voltage Vd refers to the voltage of a drainelectrode, using the potential of the source electrode as a referencepotential. In the measurement, the substrate temperature was set to −40°C., —25° C., 25° C., 85° C., 125° C., and 150° C.

FIG. 10A is a graph showing the relation between gate voltage Vg anddrain current Id at different substrate temperatures of a transistor(OSFET) where an oxide semiconductor film was used for an active layer.In the OSFET used in the measurement, an In—Ga—Zn-based oxidesemiconductor was used for the active layer, the channel length was 10nm, the channel width was 10 nm, the relative dielectric constant was4.1, and the thickness of a gate insulating film was 20 nm.

FIG. 10B is a graph showing the relation between gate voltage Vg anddrain current Id at different substrate temperatures of a transistor(SiFET) where a single crystal silicon film was used for an activelayer. The SiFET used in the measurement was an n-channel transistorwhere the channel length was 1.5 nm, the channel width was 20 nm, therelative dielectric constant was 4.1, and the thickness of a gateinsulating film was 20 nm.

In FIGS. 10A and 10B, drain current Id versus gate voltage Vg is higherat a higher substrate temperature as shown by an arrow, that is, in thefollowing order: 150° C., 125° C., 85° C., 25° C., —25° C., —40° C.However, FIGS. 10A and 10B show that the subthreshold swing of the SiFETis larger and a variation in the shift value of the SiFET is greater ata higher substrate temperature than those of the OSFET. Note that ashift value is the value of the gate voltage Vg at a drain current Id of10⁻¹² A. It is also found that a variation in the drain current Id ofthe OSFET with temperature is smaller as the gate voltage Vg in an onstate is closer to the threshold voltage as compared with the case ofthe SiFET.

FIG. 11A shows variations in shift values versus substrate temperatureof the OSFET and the SiFET, using the shift value at a substratetemperature of 25° C. as a reference value. FIG. 11A shows that avariation in the shift value of the OSFET with temperature is smallerthan that of the SiFET.

FIG. 11B shows variations of subthreshold values versus substratetemperature of the OSFET and the SiFET, using the subthreshold value ata substrate temperature of 25° C. as a reference value. FIG. 11B showsthat a variation of the subthreshold value of the OSFET with temperatureis smaller than that of the SiFET.

As seen from the example of the transistor 101 t, the drain current of atransistor including an oxide semiconductor in a channel formationregion receives an extremely small influence of temperature.Accordingly, the use of the transistor including an oxide semiconductorin a channel formation region as the semiconductor element 101 in FIG. 1or FIG. 2A makes it possible to obtain accurate temperature data of anobject to be detected.

Note that an oxide semiconductor preferably contains at least indium(In) or zinc (Zn). The oxide semiconductor preferably contains, inaddition to In and Zn, gallium (Ga) serving as a stabilizer that reducesvariations in electrical characteristics of a transistor including theoxide semiconductor. Tin (Sn) is preferably contained as a stabilizer.Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) ispreferably contained as a stabilizer. Zirconium (Zr) is preferablycontained as a stabilizer.

As another stabilizer, one or more lanthanoids such as lanthanum (La),cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium(Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) may becontained.

As the oxide semiconductor, for example, any of the following can beused: an indium oxide; a gallium oxide, a tin oxide; a zinc oxide;two-component metal oxides such as an In—Zn-based oxide, a Sn—Zn-basedoxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide,an In—Mg-based oxide, and an In—Ga-based oxide; three-component metaloxides such as an In—Ga—Zn-based oxide (also referred to as IGZO), anIn—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide,an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-basedoxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, anIn—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide,an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-basedoxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, anIn—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide,and an In—Lu—Zn-based oxide; and a four-component metal oxides such asan In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, and an In—Hf—Al—Zn-based oxide.

Note that, as an example, an In—Ga—Zn-based oxide refers to an oxidecontaining In, Ga, and Zn, and there is no limitation on the ratio ofIn, Ga, and Zn. Further, the In—Ga—Zn-based oxide may contain a metalelement other than In, Ga, and Zn. The In—Ga—Zn-based oxide hassufficiently high resistance when no electric field is applied thereto,so that the off-state current can be sufficiently reduced. Further, theIn—Ga—Zn-based oxide has high mobility.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In: Ga: Zn=2:2:1 (=2/5:2/5:1/5), or anoxide with an atomic ratio close to the above atomic ratios can be used.Alternatively, an In—Sn—Zn-based oxide with an atomic ratio ofIn:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), orIn:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or an oxide with an atomic ratio close tothe above atomic ratios may be used.

For example, with an In—Sn—Zn-based oxide, high mobility can be obtainedrelatively easily. However, even with an In—Ga—Zn-based oxide, mobilitycan be increased by reducing the defect density in the bulk.

The use of an In—Ga—Zn-based oxide or an In—Sn—Zn-based oxide amongoxide semiconductors has the following advantages: transistors havingexcellent electrical characteristics can be formed by a sputteringmethod or a wet process and thus can be mass-produced easily. Further,the oxide semiconductor an In—Ga—Zn-based oxide can be deposited even atroom temperature; thus, transistors with excellent electricalcharacteristics can be formed over a glass substrate or an integratedcircuit including silicon. Further, a larger substrate can be used.

A structure of an oxide semiconductor film is described below.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of anamorphous oxide semiconductor film, a microcrystalline oxidesemiconductor film, a polycrystalline oxide semiconductor film, a c-axisaligned crystalline oxide semiconductor (CAAC-OS) film, and the like.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystalline component. A typical example thereof is an oxidesemiconductor film in which no crystal part exists even in a microscopicregion, and the whole of the film is amorphous.

The microcrystalline oxide semiconductor film includes a microcrystal(also referred to as nanocrystal) with a size greater than or equal to 1nm and less than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor film has a higher degree of atomic order than theamorphous oxide semiconductor film. Hence, the density of defect statesof the microcrystalline oxide semiconductor film is lower than that ofthe amorphous oxide semiconductor film.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of each crystal part fits inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits a cube whose one side isless than 10 nm, less than 5 nm, or less than 3 nm. The density ofdefect states of the CAAC-OS film is lower than that of themicrocrystalline oxide semiconductor film. The CAAC-OS film is describedin detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-film observedin a direction substantially perpendicular to the sample surface (planTEM image), metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction perpendicular tothe c-axis, a peak appears frequently when 2θ is around 56°. This peakis derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis(φ scan) is performed under conditions where the sample is rotatedaround a normal vector of a sample surface as an axis (φ axis) with 2θfixed at around 56°. In the case where the sample is a single-crystaloxide semiconductor film of InGaZnO₄, six peaks appear. The six peaksare derived from crystal planes equivalent to the (110) plane. On theother hand, in the case of a CAAC-OS film, a peak is not clearlyobserved even when φ scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depends onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° is derived from the (311) plane of a ZnGa₂O₄ crystal; such a peakindicates that a ZnGa₂O₄ crystal is included in part of the CAAC-OS filmincluding the InGaZnO₄ crystal. It is preferable that in the CAAC-OSfilm, a peak of 2θ appear at around 31° and a peak of 2θ do not appearat around 36°.

In a transistor using the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline metal oxide target. When ions collide with the target, acrystal region included in the target may be separated from the targetalong an a-b plane; in other words, a sputtered particle having a planeparallel to an a-b plane (flat-plate-like sputtered particle orpellet-like sputtered particle) may flake off from the sputteringtarget. In that case, the flat-plate-like sputtered particle reaches asubstrate while maintaining their crystal state, whereby the CAAC-OSfilm can be formed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in a treatmentchamber may be reduced. Furthermore, the concentration of impurities ina deposition gas may be reduced. Specifically, a deposition gas whosedew point is −80° C. or lower, preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. When the substrateheating temperature during the deposition is increased and theflat-plate-like sputtered particles reach the substrate, migrationoccurs over the substrate, and flat surfaces of the sputtered particlesare attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the target, an In—Ga—Zn-based oxide target will bedescribed below.

The In—Ga—Zn-based oxide target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. Note that X, Y, and Z are each a givenpositive number. Here, the predetermined molar ratio of InO_(X) powderto GaO_(Y) powder and ZnO_(Z) powder is, for example, 2:2:1, 8:4:3,3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratiofor mixing powder may be determined as appropriate depending on thedesired target.

An alkali metal is not an element included in an oxide semiconductor andthus is an impurity. An alkaline earth metal is also an impurity in thecase where alkaline earth metal is not included in an oxidesemiconductor. In particular, Na among alkali metals becomes Na⁺ when aninsulating film in contact with the oxide semiconductor film is an oxideand Na diffuses into the insulating film. In addition, in the oxidesemiconductor film, Na cuts or enters a bond between a metal and oxygenwhich are contained in an oxide semiconductor. As a result, for example,degradation of electrical characteristics of a transistor, such as anormally-on state of the transistor due to shift of the thresholdvoltage in the negative direction or reduction in mobility, occurs. Inaddition, variations in electrical characteristics also occurs.Specifically, a measurement value of the Na concentration by secondaryion mass spectrometry is preferably less than or equal to 5×10¹⁶/cm³,more preferably less than or equal to 1×10¹⁶/cm³, still more preferablyless than or equal to 1×10¹⁵/cm³. In a similar manner, a measurementvalue of the Li concentration is preferably less than or equal to5×10¹⁵/cm³, more preferably less than or equal to 1×10¹⁵/cm³. In asimilar manner, a measurement value of the K concentration is preferablyless than or equal to 5×10¹⁵/cm³, more preferably less than or equal to1×10¹⁵/cm³.

FIG. 2B illustrates an example where the transistor 101 t includes onechannel formation region 121 c corresponding to one gate electrode 125,that is, the transistor 101 t has a single-gate structure. However, thetransistor 101 t may have a multi-gate structure in which a plurality ofgate electrodes electrically connected to each other are provided andthus a plurality of channel formation regions are included in one activelayer.

The transistor 101 t includes a gate electrode at least on one side ofthe active layer. Alternatively, the transistor 101 t may include a pairof gate electrodes with the active layer interposed therebetween. Whenthe transistor has a pair of gate electrodes with the active layerprovided therebetween, a signal for controlling switching is input toone of the gate electrodes, and the other of the gate electrodes may bein a floating state (i.e., electrically insulated) or a potential may besupplied to the other of the gate electrodes. In the latter case, thesame potentials may be supplied to the pair of electrodes, or a fixedpotential such as a ground potential may be supplied only to the otherof the gate electrodes. When a potential supplied to the other of thegate electrodes is controlled, the threshold voltage of the transistor101 t can be controlled.

Note that the term “connection” in this specification refers toelectrical connection and corresponds to the state in which current,voltage, or a potential can be supplied or transmitted. Accordingly, aconnection state means not only a state of direct connection but also astate of indirect connection through a circuit element such as aresistor, a diode, a transistor, or a capacitor so that current,voltage, or a potential can be supplied or transmitted.

Next, FIG. 3 illustrates another configuration example of thetemperature sensor circuit of one embodiment of the present invention.Like the temperature sensor circuit 100 in FIG. 1, the temperaturesensor circuit 100 in FIG. 3 includes the semiconductor element 101, thesemiconductor element 102, the constant current circuit 103, theconstant current circuit 104, and the amplifier circuit 105. Further,the temperature sensor circuit 100 in FIG. 3 includes an analog-digitalconvertor (ADC) circuit 106, arithmetic circuit 107, and a lookup table(LUT) 108.

The output voltage Vout output from the amplifier circuit 105 varieswith continuous change over time of the temperature of an object to bedetected. The ADC 106 has a function of obtaining and holding the outputvoltage Vout in a predetermined period, that is, a function ofperforming sampling. In addition, the ADC 106 has a function ofperforming analog-digital conversion of a sampled value of the outputvoltage Vout.

The LUT 108 stores collected data where the digital value of the Voutobtained by conversion in the ADC 106 is linked with data of thetemperature of an object to be detected. The arithmetic circuit 107 hasa function of generating a signal containing data of the temperature ofan object to be detected by arithmetic processing in accordance with thespecifications of a circuit or a device in the next stage of thetemperature sensor circuit 100; for the arithmetic processing, thedigital value of the voltage Vout obtained by conversion in the datastored in the LUT 108 is used. The voltage of the signal is output fromthe output terminal 109.

The temperature sensor circuit 100 of one embodiment of the presentinvention may further include a memory device for storing other dataused for arithmetic processing in the arithmetic circuit 107, a buffermemory device for temporarily storing data in the arithmetic processing,or the like.

The temperature sensor circuit 100 of one embodiment of the presentinvention may further include a circuit which performs signal processingon the voltage Vout. Examples of the circuit are a filter circuit and alinearization circuit. The filter circuit has a function of removingnoise from the voltage Vout. The linearization circuit has a function ofcorrecting the voltage Vout so that the value of the voltage Vout andthe physical value of an object to be detected have a linear relation.

The temperature sensor circuit 100 of one embodiment of the presentinvention may further include a current setting circuit which has afunction of correcting the difference between current values output fromthe constant current circuit 103 and the constant current circuit 104due to a difference in influence of temperature or a difference inelectrical characteristics of transistors.

Next, specific configuration examples of the constant current circuit103 and the constant current circuit 104 will be described. FIGS. 5A and5B illustrate configuration examples of the constant current circuit103. Note that the constant current circuit 104 may have the sameconfiguration as the constant current circuit 103 illustrated in FIGS.5A and 5B.

The constant current circuit 103 illustrated in FIG. 5A includes ann-channel transistor 140. One of a source terminal and a drain terminalof the transistor 140 is connected to a node supplied with the potentialVDD, and a gate electrode and the other of the source terminal and thedrain terminal of the transistor 140 is connected to a second terminalof the semiconductor element 101.

The constant current circuit 103 in FIG. 5B includes the n-channeltransistor 140 and a resistor 141. One of a source terminal and a drainterminal of the transistor 140 is connected to a node supplied with thepotential VDD, and a gate electrode and the other of the source terminaland the drain terminal of the transistor 140 is connected to oneterminal of the resistor 141. The other terminal of the resistor 141 isconnected to a second terminal of the semiconductor element 101.

Next, FIG. 4 illustrates another configuration example of thetemperature sensor circuit of one embodiment of the present invention.The temperature sensor circuit 100 in FIG. 4 includes the semiconductorelement 101, the semiconductor element 102, and the amplifier circuit105 like the temperature sensor circuit 100 in FIG. 1. The temperaturesensor circuit 100 in FIG. 4 further includes a constant voltage circuit201, a constant voltage circuit 202, a load 203, a load 204, and avoltage source 213.

The constant voltage circuit 201 is connected between a second terminalof the semiconductor element 101 and a node supplied with the potentialVDD. The constant voltage circuit 202 is connected between a secondterminal of the semiconductor element 102 and the node supplied with thepotential VDD.

Specifically, the constant voltage circuit 201 includes an n-channeltransistor 205 and a differential amplifier circuit 207. The load 203 isconnected between the node supplied with the high-level potential VDDand one of a source terminal and a drain terminal of the transistor 205.The second terminal of the semiconductor element 101 is connected to theother of the source terminal and the drain terminal of the transistor205, and a first terminal of the semiconductor element 101 is connectedto a node supplied with the low-level potential VSS. An inverting inputterminal (−) of the differential amplifier circuit 207 is connected tothe second terminal of semiconductor element 101, a non-inverting inputterminal (+) of the differential amplifier circuit 207 is connected tothe voltage source 213, and an output terminal of the differentialamplifier circuit 207 is connected to a gate electrode of the transistor205.

The constant voltage circuit 201 having the above configuration suppliesa voltage substantially equal to the voltage output from the voltagesource 213, between the first terminal and the second terminal of thesemiconductor element 101. When a current flowing through thesemiconductor element 101 flows to the load 203 through the transistor205, a voltage is generated between terminals of the load 203. Thevoltage generated between the terminals of the load 203 reflects acurrent flowing through the semiconductor element 101.

Further, specifically, the constant voltage circuit 202 includes ann-channel transistor 206 and a differential amplifier circuit 208. Theload 204 is connected between the node supplied with the high-levelpotential VDD and one of a source terminal and a drain terminal of thetransistor 206. The second terminal of the semiconductor element 102 isconnected to the other of the source terminal and the drain terminal ofthe transistor 206, and a first terminal of the semiconductor element102 is connected to the node supplied with the low-level potential VSS.An inverting input terminal (−) of the differential amplifier circuit208 is connected to the second terminal of semiconductor element 102, anon-inverting input terminal (+) of the differential amplifier circuit208 is connected to the voltage source 213, and an output terminal ofthe differential amplifier circuit 208 is connected to a gate electrodeof the transistor 206.

The constant voltage circuit 202 having the above configuration suppliesa voltage substantially equal to the voltage output from the voltagesource 213, between the first terminal and the second terminal of thesemiconductor element 102. When a current flowing through thesemiconductor element 102 flows to the load 204 through the transistor206, a voltage is generated between terminals of the load 204. Thevoltage generated between the terminals of the load 204 reflects acurrent flowing through the semiconductor element 102.

The amplifier circuit 105 has a function of amplifying a voltagedifference between the voltage generated between the terminals of theload 203 and the voltage generated between the terminals of the load 204and outputting the amplified voltage difference as the voltage Vout fromthe output terminal 110 of the amplifier circuit 105. Specifically, inthe temperature sensor circuit 100 illustrated in FIG. 4, the potentialobtained by subtracting the voltage generated between the terminals ofthe load 203 from the potential VDD and the potential obtained bysubtracting the voltage generated between the terminals of the load 204from the potential VDD are supplied to the amplifier circuit 105.

Note that the voltage generated between the terminals of the load 204contains, as data, a variation with temperature in a voltage appliedfrom the constant voltage circuit 202, a variation in electricalcharacteristics of a transistor included in the constant voltage circuit202, and the like besides the temperature of the semiconductor element102. The voltage generated between the terminals of the load 203presumably does not contain data of the temperature of the semiconductorelement 101 but contains a variation with temperature in a voltageapplied from the constant voltage circuit 201, variations in theelectrical characteristics of a transistor included in the constantcurrent circuit 201, and the like as data. Thus, assuming that there isno difference in influence of temperature, the electricalcharacteristics of the transistors, and the like between the constantcurrent circuit 201 and the constant current circuit 202, thedifferences between the constant current circuit 201 and the constantcurrent circuit 202 in influence of temperature, the electricalcharacteristics of the transistors, and the like are canceled in thevoltage Vout obtained when the amplifier circuit 105 amplifies thevoltage difference between the voltage generated between the terminalsof the load 203 and the voltage generated between the terminals of theload 204. Thus, in the temperature sensor circuit 100 of one embodimentof the present invention, a factor which affects the voltage generatedbetween the terminals of the load 204 and which is other than thetemperature of an object to be detected can be prevented frominfluencing a measurement value the voltage Vout, so that more accuratedata of the temperature of the detected object can be obtained; thefactor is, for example, a variation with temperature in a voltageapplied from the constant voltage circuit 202 to the semiconductorelement 102 or variations in the electrical characteristics of thetransistor included in the constant current circuit 202.

Note that the voltage applied to the semiconductor element 101 does notnecessarily have to be equal to the voltage applied to the semiconductorelement 102. However, when the voltages are equal to or substantiallyequal to each other, differences between the constant current circuit201 and the constant current circuit 202 in influence of temperature,the electrical characteristics of the transistors, and the like can becancelled more accurately.

As the load 203 or the load 204, for example, a resistor can be used. Asthe voltage source 213, for example, a zener diode can be used.

The temperature sensor circuit 100 of one embodiment of the presentinvention does not necessarily have to include the voltage source 213unlike in FIG. 4 and the voltage source 213 may be provided outside thetemperature sensor circuit 100.

The temperature sensor circuit 100 in FIG. 4 may further include the ADC106, the arithmetic circuit 107, and the LUT 108 included in thetemperature sensor circuit 100 in FIG. 3. The temperature sensor circuit100 in FIG. 4 may further include a filter circuit, a linearizationcircuit, or the like.

(Embodiment 2)

FIG. 6 is an example of a block diagram illustrating the configurationof a semiconductor device 300 of one embodiment of the presentinvention. The semiconductor device 300 illustrated in FIG. 6 includesthe temperature sensor circuit 100, a signal processing circuit 301, andan output device 302. Although FIG. 6 illustrates an example of theconfiguration of the semiconductor device including the temperaturesensor circuit 100 illustrated in FIG. 1, the semiconductor device ofone embodiment of the present invention may include any of thetemperature sensor circuits 100 illustrated in FIG. 2A, FIG. 3, and FIG.4.

A sensor signal output from the temperature sensor circuit 100 is inputto the signal processing circuit 301. By using the sensor signal, thesignal processing circuit 301 generates a signal for controlling theoperation of the output device 302. Specifically, the signal includesthe following: a signal for outputting temperature data which isincluded in the sensor signal to the output device 302, a signal forchanging the operation of the output device 302 in accordance with thetemperature data which is included as data in the sensor signal, or thelike.

Specific examples of the output device 302 are a display device, aprinter, a plotter, and an audio output device. For example, in the caseof using a display device as the output device 302, the temperature datacan be displayed on the display device. Further, in the case of using adisplay device as the output device 302, the temperature data can beused to control the luminance or contrast of the display device so thatthe luminance or contrast of the display device is not changed with thetemperature.

A specific configuration example of the semiconductor device 300 of oneembodiment of the present invention will be described with reference toFIG. 7.

In a liquid crystal display device, the optical property of a liquidcrystal material used for a liquid crystal layer, specifically, thetransmittance thereof with respect to an applied voltage varies withtemperature in some cases. In the semiconductor device 300 illustratedin FIG. 7, the temperature sensor circuit 100 measures the temperatureof a liquid crystal layer which is an object to be detected, and anapplication voltage to a liquid crystal element is controlled inaccordance with the temperature in a liquid crystal display deviceserving as the output device 302, whereby a change in contrast due to atemperature change can be prevented.

Specifically, the semiconductor device 300 illustrated in FIG. 7includes, like the semiconductor device 300 illustrated in FIG. 6, thetemperature sensor circuit 100, the signal processing circuit 301, andthe output device 302. Further, in FIG. 7, the output device 302includes a controller 310 and a panel 311. The panel 311 includes apixel portion 312 provided with a liquid crystal element 313 in eachpixel, and a driver circuit 314 and a driver circuit 315 which controlthe operation of the pixel portion 312. The liquid crystal element 313includes a pixel electrode whose potential is controlled by an imagesignal, a common electrode to which a predetermined reference potentialis supplied, and a liquid crystal layer to which a voltage is appliedfrom the pixel electrode and the common electrode.

A sensor signal containing data of the temperature of the liquid crystalelement 313 is input from the temperature sensor circuit 100 to thesignal processing circuit 301. In accordance with the sensor signalcontaining the temperature data, the signal processing circuit 301generates a signal for controlling the transmittance of the liquidcrystal element 313. In the output device 302, the controller 310controls, for example, a reference potential supplied to the commonelectrode in accordance with the signal for controlling thetransmittance of the liquid crystal element 313, so as to control anapplication voltage to the liquid crystal element 313.

The controller 310 has a function of processing an image signal 316 andsupplying the image signal 316 to the driver circuit 314 or the drivercircuit 315, or a function of generating a driving signal forcontrolling the operation of the driver circuit 314 and the drivercircuit 315 and supplying the driving signal to the driver circuit 314and the driver circuit 315.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 3)

A configuration example of a transistor where an oxide semiconductor isused for an active layer will be described.

A transistor 601 illustrated in FIG. 8A is a channel-etched, bottom-gatetransistor.

The transistor 601 includes a gate electrode 602 formed over aninsulating surface, a gate insulating film 603 over the gate electrode602, an oxide semiconductor film 604 serving as an active layer whichoverlaps with the gate electrode 602 with the gate insulating film 603interposed therebetween, and a conductive film 605 and a conductive film606 formed over the oxide semiconductor film 604. The transistor 601 mayfurther include an insulating film 607 formed over the oxidesemiconductor film 604, the conductive film 605, and the conductive film606.

Note that the transistor 601 illustrated in FIG. 8A may further includea gate electrode which overlaps with the oxide semiconductor film 604and is over the insulating film 607.

A transistor 611 illustrated in FIG. 8B is a channel-protective,bottom-gate transistor.

The transistor 611 includes a gate electrode 612 formed over aninsulating surface, a gate insulating film 613 over the gate electrode612, an oxide semiconductor film 614 which overlaps with the gateelectrode 612 with the gate insulating film 613 interposed therebetweenand serves as an active layer, a channel protective film 618 formed overthe oxide semiconductor film 614, and a conductive film 615 and aconductive film 616 formed over the oxide semiconductor film 614. Thetransistor 611 may further include an insulating film 617 formed overthe channel protective film 618, the conductive film 615, and theconductive film 616.

Note that the transistor 611 illustrated in FIG. 8B may further includea gate electrode which overlaps with the oxide semiconductor film 614and is over the insulating film 617.

The channel protective film 618 can prevent a portion of the oxidesemiconductor film 614 which serves as a channel formation region frombeing damaged in a later step (for example, a reduction in thickness dueto plasma or an etchant in etching). Accordingly, reliability of thetransistor 611 can be improved.

A transistor 621 illustrated in FIG. 8C is a bottom-contact, bottom-gatetransistor.

The transistor 621 includes a gate electrode 622 formed over aninsulating surface, a gate insulating film 623 over the gate electrode622, a conductive film 625 and a conductive film 626 formed over thegate insulating film 623, and an oxide semiconductor film 624 whichoverlaps with the gate electrode 622 with the gate insulating film 623interposed therebetween and is over the conductive films 625 and 626 andserves as an active layer. The transistor 621 may further include aninsulating film 627 formed over the conductive film 625, the conductivefilm 626, and the oxide semiconductor film 624.

Note that the transistor 621 illustrated in FIG. 8C may further includea gate electrode which overlaps with the oxide semiconductor film 624with the insulating film 627 interposed therebetween.

A transistor 641 illustrated in FIG. 8D is a bottom-contact, top-gatetransistor.

The transistor 641 includes a conductive film 645 and a conductive film646 formed over an insulating surface, an oxide semiconductor film 644which is formed over the conductive films 645 and 646 and serves as anactive layer, a gate insulating film 643 formed over the oxidesemiconductor film 644 and the conductive films 645 and 646, and a gateelectrode 642 which overlaps with the oxide semiconductor film 644 withthe gate insulating film 643 interposed therebetween. The transistor 641may further include an insulating film 647 formed over the gateelectrode 642.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 4)

FIG. 9 illustrates an example of part of a cross-sectional structure ofa temperature sensor circuit of one embodiment of the present invention.Note that FIG. 9 illustrates the case where the transistor 101 tincluded in the temperature sensor circuit 100 illustrated in FIG. 2Aand the transistor 140 included in the constant current circuit 103illustrated in FIG. 5A are stacked.

In this embodiment, described is the case where the transistor 140 isformed over a single crystal silicon substrate and the transistor 101 twhere an oxide semiconductor is used for an active layer is formed overthe transistor 140. In the transistor 140, a semiconductor thin film ofsilicon, germanium, or the like in an amorphous, microcrystalline,polycrystalline, or signal crystal state may be used for the activelayer.

In the case where the transistor 140 is formed using a thin siliconfilm, any of the following can be used: amorphous silicon formed by avapor phase growth method such as a plasma CVD method, or a sputteringmethod; polycrystalline silicon obtained in such a manner that amorphoussilicon is crystallized by treatment such as laser annealing; singlecrystal silicon obtained in such a manner that implantation of hydrogenions or the like into a single crystal silicon wafer is performed and asurface portion of the single crystal silicon wafer is separated; andthe like.

In FIG. 9, the semiconductor substrate 400 is provided with then-channel transistor 140.

The semiconductor substrate 400 can be, for example, a single crystalsilicon substrate having n-type or p-type conductivity, a compoundsemiconductor substrate (e.g., a GaAs substrate, an InP substrate, a GaNsubstrate, a SiC substrate, a GaP substrate, a GaInAsP substrate, or aZnSe substrate), or the like. FIG. 9 illustrates an example where asingle crystal silicon substrate having n-type conductivity is used.

The transistor 140 is electrically isolated from other semiconductorelements such as transistors by an element isolation insulating film401. The element isolation insulating film 401 can be formed by a localoxidation of silicon (LOCOS) method, a trench isolation method, or thelike.

In a region where the n-channel transistor 140 is formed, a p-well 402is formed by selective introduction of an impurity element impartingp-type conductivity. In the case where a p-channel transistor is formedusing a semiconductor substrate having p-type conductivity, an impurityelement imparting n-type conductivity is selectively introduced to aregion where the p-channel transistor is formed, so that a region calledan n-well is formed.

Specifically, the transistor 140 includes impurity regions 403 and 404which are formed in the semiconductor substrate 400 and function assource and drain regions, a gate electrode 405, and a gate insulatingfilm 406 sandwiched between the semiconductor substrate 400 and the gateelectrode 405. The gate electrode 405 overlaps with a channel formationregion formed between the impurity regions 403 and 404, with the gateinsulating film 406 interposed therebetween.

An insulating film 409 is provided over the transistor 140. Openings areformed in the insulating film 409, and wirings 410, 411, and 412 thatare in contact with the impurity region 403, the impurity region 404,and the impurity region 405, respectively, are formed in the openings.

Further, the wirings 410 and 412 are connected to a wiring 415 formedover the insulating film 409, and the wiring 411 is connected to awiring 416 formed over the insulating film 409.

An insulating film 420 is formed over the wirings 415 and 416. Anopening is formed to penetrate the insulating film 420, and a wiring 421connected to the wiring 415 is formed in the opening.

Further, in FIG. 9, the transistor 101 t is formed over the insulatingfilm 420.

The transistor 101 t includes, over the insulating film 420, asemiconductor film 430 including an oxide semiconductor; conductivefilms 432 and 433 that are provided over the semiconductor film 430 andfunction as source and drain electrodes; a gate insulating film 431 overthe semiconductor film 430 and the conductive films 432 and 433; and agate electrode 434 that overlaps with the semiconductor film 430 in aregion between the conductive films 432 and 433, with the gateinsulating film 431 interposed between the gate electrode 434 and thesemiconductor film 430.

The conductive film 433 is in contact with the wiring 421.

An insulating film 441 is provided over the transistor 101 t. Openingsare formed in the insulating film 441 and the gate insulating film 431,and a conductive film 442 in contact with the conductive film 432 in theopening, a conductive film 443 in contact with the gate electrode 434and the conductive film 433 in the opening, and a conductive film 444 incontact with the conductive film 433 in the opening are provided.

Further, an insulating film 445 is provided over the insulating film 441and conductive films 442 to 444. Openings are formed in the insulatingfilm 445, and a conductive film 446 that is in contact with theconductive film 442 in the opening and a conductive film 447 that is incontact with the conductive film 444 in the opening are provided. Theconductive films 446 and 447 preferably have high surface flatness inorder to be connected to an input terminal or a power source of anamplifier circuit later. Thus, a resin in which conductive particles aredispersed is suitable as a material of the conductive films 446 and 447.Note that since a resin has low adhesion to a solder, a conductive film448 is formed using a conductive material having high adhesion to asolder to be in contact with the conductive film 446, and a conductivefilm 449 is formed using a conductive material having high adhesion to asolder to be in contact with the conductive film 447.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

This application is based on Japanese Patent Application serial no.2012-105460 filed with the Japan Patent Office on May 2, 2012, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A temperature sensor circuit comprising: a firstconstant current circuit; a first semiconductor element where a firstvoltage is generated between a pair of terminals in accordance with afirst current supplied from the first constant current circuit; a secondconstant current circuit; a second semiconductor element where a secondvoltage is generated between a pair of terminals in accordance with asecond current supplied from the second constant current circuit; and anamplifier circuit which amplifies a difference between the first voltageand the second voltage, wherein a rate of change in the first voltagewith a first temperature of the first semiconductor element is greaterthan a rate of change in the second voltage with a second temperature ofthe second semiconductor element, wherein a value of the firsttemperature is the same as a value of the second temperature, whereinthe second semiconductor element comprises an oxide semiconductor, andwherein the first semiconductor element comprises a semiconductor otherthan the oxide semiconductor.
 2. The temperature sensor circuitaccording to claim 1 wherein the oxide semiconductor comprises In, Ga,and Zn.
 3. The temperature sensor circuit according to claim 1, whereinat least one of the first semiconductor element and the secondsemiconductor element is a diode.
 4. The temperature sensor circuitaccording to claim 1, wherein a value of the first current is equal to avalue of the second current.
 5. A semiconductor device comprising: atemperature sensor circuit which generates a signal; an output device;and a signal processing circuit which controls operation of the outputdevice using the signal, wherein the temperature sensor circuitcomprises: a first constant current circuit; a first semiconductorelement where a first voltage is generated between a pair of terminalsin accordance with a first current supplied from the first constantcurrent circuit; a second constant current circuit; a secondsemiconductor element where a second voltage is generated between a pairof terminals in accordance with a second current supplied from thesecond constant current circuit; and an amplifier circuit whichamplifies a difference between the first voltage and the second voltageand generates the signal, and wherein a rate of change in the firstvoltage with a first temperature of the first semiconductor element isgreater than a rate of change in the second voltage with a secondtemperature of the second semiconductor element, wherein a value of thefirst temperature is the same as a value of the second temperature,wherein the second semiconductor element comprises an oxidesemiconductor, and wherein the first semiconductor element comprises asemiconductor other than the oxide semiconductor.
 6. The semiconductordevice according to claim 5, wherein the output device is a liquidcrystal display device.
 7. The semiconductor device according to claim6, wherein each pixel of the liquid crystal display device includes aliquid crystal element, and wherein the temperature depends on atemperature of a liquid crystal layer included in the liquid crystalelement.
 8. The semiconductor device according to claim 7, wherein thesignal processing circuit is configured to adjust a voltage applied tothe liquid crystal element using the signal to control operation of theoutput device.
 9. The semiconductor device according to claim 5, whereinthe oxide semiconductor comprises In, Ga, and Zn.
 10. The semiconductordevice according to claim 5, wherein at least one of the firstsemiconductor element and the second semiconductor element is a diode.